1. Technical Field
The present disclosure relates to a control device for rectifiers of switching converters and, in particular, of resonant converters.
2. Description of the Related Art
Resonant converters are a vast class of forced switching converters characterized by the presence of a resonant circuit that actively participates in determining the input-output power flow. In such converters a circuit consisting of a power switching bridge or half-bridge (typically of power MOSFETs), powered by DC voltage, generates a square wave voltage that is applied to a resonant circuit tuned to the fundamental frequency of the square wave. In this manner, because of its selective characteristics, the resonant circuit responds principally to this fundamental component and to a negligible degree to higher-order harmonics.
It follows that the circulating power can be modulated by varying the frequency of the square wave while maintaining the duty cycle constant at 50%, and that, according to the configuration of the resonant circuit, the currents or voltages associated with the power flow will have a pattern that is sinusoidal or piecewise sinusoidal. These voltages and currents are rectified and filtered so as to supply DC power to the load.
In offline applications, for reasons tied to safety regulations, the rectifier-filter system that supplies power to the load is coupled to the resonant circuit by means of a transformer that provides the isolation between source and load required under the aforesaid regulations. As in all isolated network converters, in this case as well it is customary to distinguish between a primary side (i.e., relating to the primary winding of the transformer) connected to the input source and a secondary side (i.e., relating to the secondary winding—or secondary windings—of the transformer) which supplies power to the load via the rectifier-filter system.
At present, one of the resonant converters most widely used is the LLC resonant converter, especially the half-bridge version. This name derives from the fact that the resonant circuit employs two inductors and one capacitor. The principle schematic of the half-bridge version is shown in FIG. 1, where a half-bridge of two transistors M1, M2 powered by an input voltage Vin and driven by a device 1, powers a series comprising a capacitor C, an inductance Ls and an inductance Lp, with a transformer 10 connected in parallel with the inductance Lp.
The transformer has a secondary winding with a center tap connected to ground GND, whereas the ends of the secondary winding are connected to rectifier diodes D1 and D2 having the cathodes connected together and to a parallel of a capacitor C1 and a resistance R, across which the output voltage Vout is present.
This converter, in addition to the typical advantages of resonant converters (waveforms without steep fronts, low switching losses of the power switches due to “soft” switching), has substantial advantages over converters that employ resonant circuits with only two reactive elements. In fact, the LLC converter is capable of working in a vast range of operating conditions with respect to input voltage and output current, including no load conditions, with a relatively small frequency variation; it has the possibility of achieving “soft” switching operations with all power switches in all operating conditions with respect to input voltage and output current. In fact, the power MOSFETs on the primary side have zero voltage turn-on (ZVS—Zero Voltage Switching), hence zero associated losses; whereas the rectifiers on the secondary side have zero current turn-on and turn-off (ZCS, Zero Current Switching) and hence with no reverse recovery and the phenomena associated therewith. The turn-off switching losses of the primary side power MOSFETs are also rather low. Additionally, a further advantage is magnetic integration, i.e., the possibility of combining all of the magnetic devices (inductances and transformer) in a single physical component.
As a consequence of such properties, these resonant converters are characterized by a high conversion efficiency (>95% is easily achievable), an ability to work at high frequencies, low generation of EMI (Electro-Magnetic Interference) and, finally, a high power density (which means the possibility of constructing conversion systems of reduced volume).
In current types of converter circuits, a high conversion efficiency and high power density are required, as in the case, for example, of the AC-DC adaptors of notebooks. LLC resonant converters are at present the converters that best meet such requirements.
However, the maximum efficiency achievable is limited by the losses in the rectifiers on the secondary side of the converter, which account for over 60% of total losses.
It is known that in order to significantly reduce the losses connected to secondary rectification, recourse can be made to the so-called “synchronous rectification” technique, in which rectifier diodes are replaced by power MOSFETs, with a suitably low on-resistance, such that the voltage drop across it is significantly lower than that across the diode; and they are driven in such a manner as to be functionally equivalent to the diode. This technique is widely adopted in traditional converters, especially in flyback and forward converters, for which there also exist commercially available dedicated integrated control circuits. There is an increasingly pressing need to adopt this technique in resonant converters as well, in particular in LLC converters, in order to enhance their efficiency as much as possible.
FIG. 2 shows the converter of FIG. 1 in the version with secondary synchronous rectifiers; in this case, in the place of diodes D1 and D2 there are two transistors T1 and T2, suitably driven by two signals G1 and G2 and connected between the terminals of the two parts of the center-tapped secondary winding connected to ground GND, while the parallel of C1 and R is disposed between the center tap of the secondary winding and ground GND. From a functional viewpoint there is no difference, as compared to the schematic in FIG. 1.
To drive the power MOSFETs T1 and T2 as synchronous rectifiers in a resonant converter use is sometimes made of methods borrowed from traditional PWM controlled converters, based, that is, on the “self-driven” approach, in which the drive voltage of the synchronous rectifiers is obtained through the auxiliary windings of the transformer, and the “primary-driven” approach, in which the same signal which drives the primary side power MOSFET gates is used to drive the gates of the synchronous rectifiers on the secondary side. Both methods present major drawbacks. In the case of the self-driven approach, the drive voltage does not have steep fronts which determine fast switching (necessary especially at turn-off) so that, due to the delays, current reversals can be observed in the synchronous rectifiers. These reversals, by acting as a dummy load, discharge the output capacitors, thereby increasing the output voltage ripple and impairing efficiency at medium-low loads, a parameter that is of hardly negligible importance given the most recent legislation regarding the reduction of consumption. In addition, the current regime in the transformer and in the resonant circuit is altered and in certain conditions a loss of ZVS can be observed, with consequences that may range from a moderate increase in power dissipation in the primary-side power MOSFETs to the destruction thereof due to triggering of the parasitic bipolar transistor intrinsic to the structure of the power MOSFETs.
In the case of the “primary-driven” approach, the converter functions correctly as long as the conduction of current in each synchronous rectifier occupies the entire switching half-period (CCM, Continuous Conduction Mode). Otherwise, that is, if the conduction of current to the secondary winding occupies only a fraction of the switching half-period (DCM, Discontinuous Conduction Mode), the converter will no longer work correctly. In fact, if the same signal is used for the primary-side MOSFETs and the synchronous rectifiers, the latter will remain turned on even if the current falls to zero before completion of the half-cycle, resulting in a reversal of current along with the aforesaid drawbacks. This constitutes a severe limitation to the operating capabilities of the converter. In order to avoid working in DCM, not only is there the constraint of working within a narrow operating range, but it is also necessary for the load never to fall below a certain minimum value because, like all converters, also the LLC resonant converter tends to work in DCM, in the sense described just above, under a low load.
Recently, more refined techniques have been developed with the aim of improving the drive logic of secondary synchronous rectifiers in this particular topology. Examples of these techniques are described in U.S. Pat. No. 7,184,280 and U.S. Pat. No. 7,193,866. In both cases the driving signals for the primary side power MOSFETs and for the synchronous rectifiers are generated by a single control circuit, which establishes their mutual relation. The fundamental drawback of such methods is that one or the other of the driving signals must cross the isolation barrier between the primary and secondary sides and hence the use of an additional transformer is necessary. In addition to this, neither of the two methods takes into account the fact that the current across the secondary diodes (and thus also in the synchronous rectifiers, if appropriately controlled) may be null not only in the final part of each switching half-cycle but also in the initial part.
A final aspect, which is taken marginally into consideration in U.S. Pat. No. 7,184,280, is the advisability of suspending the synchronous rectification when the output current is low and entrusting the secondary rectification function either to the body diodes of the power MOSFETs used as synchronous rectifiers or Schottky diodes connected in anti-parallel with the synchronous rectifiers. In fact, with low currents, the reduction in losses associated with conduction across the turn-on resistance (as compared to the losses across the diode) is cancelled out by the loss of power used to drive the synchronous rectifiers.
Recently, IR has released to the market a device specific for synchronous rectification control in LLC resonant half-bridge converters. It uses a control methodology that enables the synchronous rectifier MOSFETs to be driven without any connection with the driving signals of the primary-side switches. The drain-to-source voltage of each MOSFET is sensed and, when it falls below a threshold the MOSFET is turned on. When the drain-to-source voltage exceeds a pre-determined threshold, the MOSFET is turned off. Fixed blanking times, during which the sensing circuit (or its output) is ignored, are provided after turn-on and turn-off to prevent multiple switching. Except that simultaneous conduction of the two MOSFETS is prevented, there is no cross-coupling logic that is concerned with ensuring a symmetrical behavior of the two MOSFETs.
For reasons that will be clear after the detailed description of the methodology proposed in this disclosure, the drawback in this method is a short conduction time that is too short for the synchronous rectifier MOSFETs at medium load, which impairs efficiency in this condition, and the absence of any provision for disabling synchronous rectifier MOSFETs at low load, where synchronous rectification may be detrimental as far as efficiency is concerned.